1. Field of the Invention
The invention relates in general to an apparatus for the ring-back constriction, and more particularly to an apparatus for the ring-back constriction for a high-frequency low-swing transmission bus.
2. Description of the Related Art
Computers today run at a much higher clock frequency, and thus consume more power. The high-frequency low-swing transmission bus in the computer becomes popular because the computer is faster and more power save. Gunning Transceiver Logic Plus (GTL+) bus is one of the high-frequency low-swing transmission buses. In a computer, GTL+ buses are used between CPU and the north bridge chipset as transmission buses. A GTL+ bus includes one transmission line with two termination resistors to a termination voltage of high level. If the impedance of the transmission line does not match with the termination resistor, a reflection occurs in the bus. A reflective wave superimposes with the original signal and accordingly affects the wave shape of the signal. This phenomenon is known as a ring-back effect. The termination resistor reduces the ring-back effect and pulls high the voltage of the bus for stabilizing the signal.
The comparator compares the signal on the transmission line with a reference voltage and accordingly outputs a comparison signal. When the line signal is lower in magnitude than the reference voltage, the line signal is determined as low, and the comparator outputs a low-level comparison signal. When the line signal is higher in magnitude than the reference voltage, the line signal is determined as high, and the comparator outputs a high-level comparison signal. Accordingly, the correctness of the line signal received is ensured. However, if the output node of the transmission line is positioned at the end of the transmission line, a termination resistor is required to match the impedance of the transmission line. The termination resistor constricts the ring-back effect and can pull high the voltage of the transmission line.
FIG. 1 is a diagram of a well-known apparatus for the ring-back constriction. The apparatus includes a termination resistor R and a comparator 102. The comparator 102 receives the signal Vi on the transmission line L of a GTL+ bus. The comparator 102 compares the line signal Vi with a reference voltage Vref and outputs a comparison signal Vc. The comparison signal Vc determines the line signal Vi to be at either high or low level. In the figure, the voltage Vtt is the termination voltage of the GTL+ bus. Generally, the voltage Vtt is 1.5 volt, and the reference voltage is 1 volt.
When the line signal Vi is high, the voltage difference between the two ends of the termination resistor R is almost zero, and thus the termination resistor R consumed almost no power. When the line signal Vi transits from high to low, the ring-back effect can be constricted due to the impedance match between the termination resistor end and the transmission line L. However, a current flows through the termination resistor R when the line signal Vi is low, and the power consumption increases. In addition, each transmission line of the GTL+ bus needs termination resistors outside the chipset and thus the cost is increased.
FIG. 2 is another well-known apparatus for the ring-back constriction. The apparatus uses variable resistors as termination resistors. The advantage is that the power consumption is reduced. That is because when the line signal is low, the resistance of the variable resistor is turned to extremely high. The apparatus includes a termination controller 202, a termination variable resistor Rt, and a comparator 203. When the line signal Vi is high, the resistance of the termination variable resistor Rt becomes low according to the termination control signal St generated by the termination controller 202. The ring-back effect is thus constricted due to the impedance match. The signal on the transmission line L is also stabilized because the voltage of the transmission line L is pulled high by the voltage Vtt through the termination variable resistor Rt. In addition, almost no power is consumed at the termination variable resistor Rt because the voltage difference at the two ends of the termination variable resistor Rt is almost zero. When the line signal Vi is low, the resistance of the termination variable resistor Rt becomes high according to the termination control signal St generated by the termination controller 202. Also, almost no power is consumed at the termination variable resistor Rt because the resistance of the termination variable resistor Rt is high. The termination variable resistor Rt is implemented by a transistor, and the apparatus is included in the chipset.
However, in this prior art, the impedance match is not good enough if the termination variable resistor Rt does not have a good linear I-V curve. The line signal Vi undershoot and accordingly a ring-back occurs. Detailed description about the effects of the ring-back constriction using fixed resistors (as in FIG. 1) and variable resistors (as in FIG. 2) will be discussed in the following paragraph.
FIG. 3A is the transition diagram of the line signal Vi stated in FIG. 1. As shown, when the line signal Vi transits, the ring-back effect can be efficiently constricted. In the figure, the bit time tb is the time needed to transfer a bit. FIG. 3B is the transition diagram of the line signal Vi stated in FIG. 2. The ring-back constriction is not as good as that shown in FIG. 3A because the termination variable resistor Rt is implemented by a transistor in integrated circuits; while the resistor shown in FIG. 1 is a common resistor. The node A is an undershooting point, and the node B is a ring-back point. If the voltage difference between the ring-back value at node B and the reference voltage Vref is less than 0.2 volt, the reading of the line signal will possibly be wrong. When the working frequency of the GTL+ bus is 100 MHz, the apparatus shown in FIG. 2 can work normally without errors. However, when the working clock frequency of the GTL+ bus is higher, such as 133 MHz, ring-back effects become more obvious and thus reading errors occur.